PHP decrement NULL value issue

Question

I have noticed if a value is null i can increment it by one using ++$value but its not true about decrement , meaning --$value would return null , why?

$value = null;
echo ++$value; // 1
echo --$value; // null (I'm expecting -1)

Answer 1

Ref# language.operators.increment.php

Note: The increment/decrement operators only affect numbers and strings. Arrays, objects and resources are not affected. Decrementing NULL values has no effect too, but incrementing them results in 1.